The Pacific/North American (PNA) pattern is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extratropics, appearing in all months except June and July. The PNA pattern reflects a quadripole pattern of 500 millibar height anomalies, with anomalies of similar sign located south of the Aleutian Islands and over the southeastern United States. Anomalies with sign opposite to the Aleutian center are located in the vicinity of Hawaii, and over the intermountain region of North America (central Canada) during the winter and fall.
The spatial scale of the PNA pattern is most expansive in winter. During this period, the Aleutian center spans most of the northern latitudes of the North Pacific. In spring, the Aleutian center contracts and becomes confined primarily to the Gulf of Alaska. However, the subtropical center near Hawaii reaches maximum amplitude during the spring. The PNA pattern then disappears during June and July, but reappears in the late summer and fall. During this period, the midlatitude centers become dominant and appear as a wave pattern emanating from the eastern North Pacific. The subtropical center near Hawaii is weakest during this period.
The PNA index is obtained by projecting the PNA loading pattern to the daily anomaly 500 millibar height field over 0-90°N. The PNA loading pattern has been chosen as the second mode of a Rotated EOF analysis using monthly mean 500 millibar height anomaly data from 1950 to 2000 over 0-90°N latitude. For more information, please see CPC's Pacific-North American Pattern website.